Despite the broad range of interest and applications, controls on calcite surface charge in aqueous solution, especially
at conditions relevant to natural systems, remain poorly understood. The primary data source to understand
calcite surface charge comprises measurements of zeta potential. Here we collate and review previous
measurements of zeta potential on natural and artificial calcite and carbonate as a resource for future studies,
compare and contrast the results of these studies to determine key controls on zeta potential and where uncertainties
remain, and report new measurements of zeta potential relevant to natural subsurface systems.
The results show that the potential determining ions (PDIs) for the carbonate mineral surface are the lattice ions
Ca2+, Mg2+ and CO3
2−. The zeta potential is controlled by the concentration-dependent adsorption of these ions
within the Stern layer, primarily at the Outer Helmholtz Plane (OHP). Given this, the Iso-Electric Point (IEP) at
which the zeta potential is zero should be expressed as pCa (or pMg). It should not be reported as pH, similar
to most metal oxides.
The pH does not directly control the zeta potential. Varying the pH whilst holding pCa constant yields constant
zeta potential. The pH affects the zeta potential only by moderating the equilibrium pCa for a given CO2 partial
pressure (pCO2). Experimental studies that appear to yield a systematic relationship between pH and zeta potential
are most likely observing the relationship between pCa and zeta potential, with pCa responding to the change
in pH. New data presented here show a consistent linear relationship between equilibrium pH and equilibrium
pCa or pMg irrespective of sample used or solution ionic strength. The surface charge of calcite is weakly dependent
on pH, through protonation and deprotonation reactions that occur within a hydrolysis layer immediately
adjacent to the mineral surface. The Point of Zero Charge (PZC) at which the surface charge is zero could be
expressed as pH, but surface complexation models suggest the surface is negatively charged over the pH range
5.5–11.
Several studies have suggested that SO4
2− is also a PDI for the calcite surface, but new data presented here indicate
that the value of pSO4 may affect zeta potential only by moderating the equilibrium pCa. Natural carbonate typically
yields a more negative zeta potential than synthetic calcite, most likely due to the presence of impurities
including clays, organic matter, apatite, anhydrite or quartz, that yield a more negative zeta potential than
pure calcite. New data presented here show that apparently identical natural carbonates display differing zeta
potential behaviour, most likely due to the presence of small volumes of these impurities. It is important to ensure
equilibrium, defined in terms of the concentration of PDIs, has been reached prior to taking measurements.
Inconsistent values of zeta potential obtained in some studies may reflect a lack of equilibration.
The data collated and reported here have broad application in engineering processes including the manufacture
of paper and cement, the geologic storage of nuclear waste and CO2, and the production of oil and gas